High precision and low power electromechanical system design is required for future direct access storage devices (DASDs), particularly targeted for mobile computer systems. Without limiting operating vibration performance, servo architecture of a DASD should be capable of providing low positioning error without exceeding a competitive power budget.
Conventional methods attempt to achieve minimum positioning error by reducing the repeatable runout (RRO) component, that is any deviation from an ideal circular track, due to for example mechanical tolerances or servo writing, at the manufacturing level by imposing very severe manufacturing tolerance requirements, or by implementing gain enhancing algorithms, such as feedforward or narrow band filters, at RRO spectral frequencies. These approaches either require high cost manufacturing methods or increased track following voice coil motor (VCM) power. In track following mode a disk actuator servo is designed to minimize head positioning error in the presence of repeatable and non-repeatable runout components (NRRO) that are present in the position error signal (PES). Traditional methods attempt to minimize either the RRO component at source by refining the manufacturing process or suppress RRO by implementing high gain servo schemes. Minimizing RRO at the source has a manufacturing cost penalty while a servo solution that effectively generates frequency specific current to the voice coil motor (VCM) has a power penalty.
A typical DASD servo as shown in FIG. 1, has a PES generating block 10, servo computation block 12, and a digital-to-analog converter (DAC) block 14. FIG. 1 also shows a feedforward scheme including a misposition correction signal generator 16 augmenting the conventional servo to improve RRO track following capability. The output of generator 16 is added to the output of block 12 at a summing node 18. The added output of node 18 is then provided to DAC block 14. The analog output of DAC block 14 is provided to a current driver 20 which provides current to the VCM (not shown in FIG. 1) of the actuator block 22, which represents the mechanical components that control the position of a transducer (not shown) which interacts with a data storage medium (also not shown in FIG. 1).
In the conventional servo architecture the control signal is first computed from PES signal derived from the data surface, and then the control signal generated by the servo computation block 12 is modified by the misposition correction signal (also called a feedforward signal) from block 16. The PES generating block derived from the data surface invariably contains both RRO and NRRO components, represented by block 24. It will be understood that the signal sent to block 10 is the relative difference between a head position signal received from the block 22 and the runout components of a track as represented by block 24. While a subtraction node 25 is shown, it will be understood that this is a theoretical node because it is only the difference, which is generated by block 10 that is available when the recording head signal is processed for use by the servo loop.